Plasmids, conjugative or non-conjugative, determining antibiotic resistances, colicin production or other phenotypic properties, will be introduced into Salmonella typhimurium strains carrying revertible mutations causing auxotrophy, and the effects of plasmids of various compatibility groups on rates of spontaneous mutation and on the mutagenic effect of exposure to UV or visible light, or to known or suspected chemical mutagens and carcinogens will be measured. The ability of plasmids to reduce the lethal effect of such exposure will also be tested, in wild-type and in repair-deficient hosts. Additional auxotrophy mutations will be introduced into Ames-test strains and into new tester strains derived from an S. typhimurium line not carrying prophages Fels 1 and Fels 2.